1. Field of the Invention
The present invention relates to an electromagnetic valve provided with a valve element for opening and closing a hydraulic fluid passage.
2. Description of the Related Art
Generally, electromagnetic valves (solenoid valves) for opening and closing a hydraulic fluid passage are broadly divided into a normally close type electromagnetic valve which is normally closed but opens by excitation of a fixed core when current is supplied to a coil, and a normally open type electromagnetic valve which is normally open but closes by excitation of a fixed core when current is supplied to a coil. These electromagnetic valves are provided with a valve seat for defining a hydraulic fluid passage and a valve element capable of contacting with or separating from the valve seat. The valve element can close the hydraulic fluid passage by contacting with the valve seat, and open the hydraulic fluid passage by separating from the valve seat.
FIG. 5 shows an example of such an electromagnetic valve, which is provided with a spherical valve element 300 fixed in a plunger 240 (See Japanese Patent Examined Publication JP-B-2745339). The spherical valve element 300 is attached to the plunger 240 by first press-fitting the spherical valve element 300 into a circular recess 260 disposed at a tip portion of the movable plunger 240 (a movable core in a normally close type electromagnetic valve; a retainer in a normally open type electromagnetic valve), and then by fixing the tip portion of the plunger 240 against the spherical valve element 300. Since such an electromagnetic valve is provided with a small-diameter valve element, a seating area diameter on the valve seat 420 can be reduced, thereby permitting energy saving and miniaturization of a coil for activating the valve element.
However, since the tip portion of the plunger 240 has to be fixed to enclose the spherical valve element 300, there is inevitably a technical limit in reducing the diameter of the spherical valve element 300, because of interference with the valve seat element 400. To be more specific, the tip portion of the plunger 240 has to enclose more than half a height of the spherical valve element 300, and furthermore the tip portion has to have a sufficient thickness for the fixing process. Therefore, in a case where the spherical valve element 300 becomes smaller than a certain limit, the tip portion of the plunger 240 interferes with the valve seat element 400.
Also, FIG. 6 shows a valve element 302 made of a PEEK (polyether-etherketon) resin provided with a valve element and a plunger formed in a unified body (See Japanese Patent Unexamined Publication JP-A-2002-347597). The valve element 302 unified with the plunger includes a shaft portion 350 of a circular column shape and a semi-spherical sealing portion 360 formed at a tip thereof. However, forming such a valve element 302 requires processing precision applicable to a plunger which slides back and forth in a fixed core, as well as to a valve element which has to ensure a sealing effect be making contact with a valve seat with reduced operational noise. Therefore, process control is required for fulfilling the precision in both aspects, however such control was quite troublesome.